1. Field of the Invention
The present invention relates to a micro mirror unit to be used in e.g. an optical switching device for switching optical paths provided by optical fibers.
2. Description of the Related Art
In recent years, optical communications technology is utilized widely in a variety of fields. In the optical communications, optical fibers serve as a medium through which optical signals are passed. When the optical signal passing through a given optical fiber is switched to another optical fiber, so-called optical switching devices are used. In order to achieve high quality optical communications, the optical switching device must have such characteristics as high capacity, high speed and high reliability in switching action. In view of these, micro mirror units manufactured by micro-machining technology are very popular as a switching element to be incorporated in the optical switching device. The micro mirror units enable the switching operation without converting optical signals into electric signals between the optical paths on the input side and the output side of the optical switching device. This is advantageous to achieving the above-mentioned characteristics.
Optical switching devices utilizing micro mirror units manufactured by micro-machining technologies are disclosed, for example, in International Publication WO00/20899, and the article Fully Provisioned 112xc3x97112 Micro-Mechanical Optical Crossconnect with 35.8Tb/sec Demonstrated Capacity (Proc. 25th Optical Fiber Communication Conf. Baltimore. PD12(2000).
FIG. 18 outlines an ordinary optical switching device 500. The optical switching device 500 includes a pair of micro mirror arrays 501, 502, an input fiber array 503, an output fiber array 504, and a plurality of micro lenses 505, 506. The input fiber array 503 includes a predetermined number of input fibers 503a. The micro mirror array 501 is provided with the same number of micro mirror units 501a each corresponding to one of the input fibers 503a. Likewise, the output fiber array 504 includes a predetermined number of input fibers 504a. The micro mirror array 502 is provided with the same number of micro mirror units 502a each corresponding to one of the output fibers 504a. Each of the micro mirror units 501a, 502a has a mirror surface to reflect light. The orientation of the mirror surface is controllable. Each of the micro lenses 505 faces an end of a corresponding input fiber 503a. Likewise, each of the micro lenses 506 faces an end of a corresponding output fiber 504a. 
In transmitting optical signals, lights L1 coming out of the input fiber array 503a pass through the corresponding micro lenses 505, thereby becoming parallel to each other and proceeding to the micro mirror array 501. The lights L is reflected on their corresponding micro mirror units 501a respectively, thereby directed toward the micro mirror array 502. The mirror surfaces of the micro mirror unit 501a are oriented, in advance, in appropriate directions so as to direct the light L1 to enter the desired micro mirror units 502a. Then, the light L1 is reflected on the micro mirror units 502a, and thereby directed toward the output fiber array 504. The mirror surfaces of the micro mirror units 502a are oriented, in advance, in appropriate directions so as to direct the light L1 to the desired output fibers 504a. 
As described, according to the optical switching device 500, the light L1 coming out of the input fibers 503a reaches the desired output fibers 504a due to the reflection by the micro mirror arrays 501, 502. In this manner, a given input fiber 503a is linked to the relevant output fiber 504a in a one-to-one relationship. By appropriately changing the orientation, of the micro mirror units 501a, 502a, switching can be performed and the light L1 can be directed toward the selected output fiber 504a. 
FIG. 19 outlines another ordinary optical switching device 600. The optical switching device 600 includes a micro mirror array 601, a fixed mirror 602, an input-output fiber array 603, and a plurality of micro lenses 604. The input-output fiber array 603 includes a number of input fibers 603a and output fibers 603b. The micro mirror array 601 includes the same number of micro mirror units 601a each corresponding to one of the fibers 603a, 603b. Each of the micro mirror units 601a has a mirror surface for reflection of light, the orientation of the mirror surfaces being controllable. Each of the micro lenses 604 faces an end of a corresponding one of the fibers 603a, 603b. 
In transmitting optical signals, light L2 coming out of the input fiber 603a passes through the corresponding micro lens 604 and is directed toward the micro mirror array 601. The light L2 is then reflected by a corresponding first micro mirror unit 601a, and thereby directed toward the fixed mirror 602, reflected by the fixed mirror 602, and then enters a corresponding second micro mirror unit 601a. The mirror surface of the first micro mirror unit 601a is oriented, in advance, in a predetermined direction so as to direct the light L2 to enter a selected one of the micro mirror units 601a. Then, the light L2 is reflected on the second micro mirror unit 601a, and thereby directed toward the input-output fiber array 603. The mirror surface of the second micro mirror unit 601a is oriented, in advance, in a predetermined direction so as to direct the light L2 to enter a predetermined one of the output fibers 603b. 
As described, according to the optical switching device 600, the light L2 coming out of the input fiber 603a reaches the desired output fiber 603b due to the reflection by the micro mirror array 601 and the fixed mirror 602. In this manner, a given input fiber 603a is linked to the relevant output fiber 603b in a one-to-one relationship. With this arrangement, by appropriately changing the orientation of the first and the second micro mirror units 601a, switching can be performed and the light L2 can be directed toward the selected output fiber 603b. 
According to the optical switching devices 500, 600 as described above, the number of fibers increases with increase in the size of optical communications network. This means that the number of micro mirror units, or mirror surfaces, incorporated in the micro mirror array also increases. With a greater number of mirror surfaces, a greater amount of wiring is required to drive the mirror surfaces and therefore, an increased amount of area must be provided for the wiring per micro mirror array. If the mirror surfaces and the wiring pattern are to be formed in the same substrate, an increased amount of wiring requires an increased pitch between the mirror surfaces. As a result, the substrate itself or the micro mirror array as a whole must be big. In addition, an increase in the number of mirror surfaces tends to make it difficult to form the mirror surfaces together with the wiring pattern in the same substrate.
The present invention has been proposed under the circumstances described above. It is therefore an object of the present invention to provide a micro mirror unit capable of reducing the size-increasing tendency resulting from the increase in the number of mirror surfaces.
According to a first aspect of the present invention, there is provided a micro mirror unit provided with: a micro mirror substrate that includes a moving part, a first frame and torsion bars connecting the moving part to the frame, the moving part being provided with a mirror-formed portion; a wiring substrate formed with a wiring pattern; and an electroconductive spacer for electrically connecting the frame to the wiring pattern and for spacing the micro mirror substrate and the wiring substrate apart from each other.
With the above arrangement, the moving part (carrying a mirror portion) is provided in one substrate, and the wiring necessary to operate the moving part in another. This allows the micro mirror unit to be smaller than when the moving part and the wiring are provided on the same substrate. With the use of the electroconductive spacer, the spaced mirror and wiring substrates can be electrically connected to each other. Further, since the mirror substrate (in which the moving part is provided) is spaced apart from the wiring substrate by the spacer, the moving part can pivot properly without interfering with the wiring substrate.
According to a second aspect of the present invention, there is provided a micro mirror unit provided with: a micro mirror substrate formed integral with a plurality of micro mirror elements each including a moving part, a frame and torsion bars connecting the moving part to the frame, the moving part being provided with a mirror-formed portion; a wiring substrate formed with a wiring pattern; and an electroconductive spacer for electrically connecting the frame to the wiring pattern and for spacing the micro mirror substrate and the wiring substrate apart from each other.
Preferably, the electroconductive spacer may consist of a single bump or a plurality of stacked bumps.
Preferably, the electroconductive spacer may be connected to at least one of the wiring pattern and the frame via an electrode pad or electroconductive adhesive.
Preferably, the electroconductive spacer and the electrode pad may be fused to each other or press-contacted with each other.
Preferably, the wiring substrate may have a first surface facing the micro mirror substrate, and the first surface may be formed with a retrieved portion for accommodation of the moving part.
Preferably, the wiring substrate may have a second surface opposite to the first surface, and the second surface may be formed with part of the wiring pattern.
Preferably, the wiring substrate may include an electrical conductor penetrating through the wiring substrate for electrical connection between the wiring pattern formed in the first surface and the wiring pattern formed in the second surface.
Preferably, the micro mirror substrate and the wiring substrate may be fixed to each other by an adhesive.
Preferably, the micro mirror unit of the present invention may further include an additional spacer intervening between the frame and the wiring substrate. The additional spacer may be a bump.
Preferably, the moving part may be provided with a first comb-like electrode, while the frame may be provided with a second comb-like electrode for operation of the moving part by static electric force generated between the first and the second comb-like electrodes.
Preferably, the moving part may include a relay frame connected to the first-mentioned frame via the torsion bars, a mirror-formed portion spaced from the relay frame, and relay torsion bars connecting the relay frame and the mirror-formed portion to each other. The relay torsion bars may extend in a direction crossing the direction in which the torsion bars extend.
Preferably, the mirror-formed portion may include a third comb-like electrode, while the relay frame may include a fourth comb-like electrode for operation of the mirror-formed portion by static electric force generated between the third and the fourth comb-like electrodes.
Preferably, the micro mirror substrate may include a plurality of divisions insulated from each other by at least one of an insulating film and a gap, part of the divisions being electrically connected to the electroconductive spacer.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.